A 1200/2400 bps coding suite based on MELP

Abstract

This paper presents key algorithm features of the future NATO narrow band voice coder (NBVC), a 1.2/2.4 kbps speech coder with noise preprocessor based on the MELP analysis algorithm. At 1.2 kbps, the MELP parameters for three consecutive frames are grouped into a superframe and jointly quantized to obtain high coding efficiency. The inter-frame redundancy is exploited with distinct quantization schemes for different unvoiced/voiced (U/V) frame combinations in the superframe. Novel techniques used at 1.2 kbps include pitch vector quantization using pitch differentials, joint quantization of pitch and U/V decisions and LSF quantization with a forward-backward interpolation method. A new harmonic synthesizer is introduced for both rates which improves the reproduction quality. Subjective test results indicate that the 1.2 kbps speech coder achieves quality close to the existing federal standard 2.4 kbps MELP coder.